This application claims the priority benefit of Taiwan application Ser. No. 90113549, filed on Jun. 5, 2001.
1. Field of Invention
The present invention relates to a bonding pad structure. More particularly, the present invention relates to a bonding pad structure having detached current conduction regions and mechanical support regions.
2. Description of Related Art
In the front stage of fabricating semiconductor devices, a plurality of optical masks is used to pattern out the active regions, gate structures, metallic layers, source/drain contacts, circuit pattern of multi-level interconnects and bonding pad windows. Due to the rapid increase in the level of integration of semiconductor devices, functional capacity and data processing speed, the number of signaling points on a semiconductor component increases considerably. As the number of contact points increases, the number of corresponding bonding pads required is also increased. After the formation of bonding pads, the integrated circuit chip must be packaged. In other words, the signal points and bonding pads on the silicon chip must connect electrically with a lead frame via metallic wires, a process known as wire-bonding. A wire-bonding operation links each bonding pad on a semiconductor chip with an inner lead of the lead frame using a fine metallic wire (30-50xcexcm). Hence, electrical signal generated inside the semiconductor chip can be transmitted to circuits outside the package. The bonding pad on the semiconductor chip serves as a first bonding point while the inner lead of the lead frame serves as a second bonding point. During wire bonding, one end of a metallic wire is melted into a spherical blob and then the spherical blob is pressed onto the bonding pad to form a weld with the aid of an ultrasonic vibration. The metallic wire is pulled along a pre-defined path towards a corresponding inner lead position on the lead frame. Thereafter, the other end of the wire is bonded to the inner lead. Finally, excess metallic wire is pulled off from the bonded inner lead. By repeating the aforementioned wire bonding process, the entire package is connected. Because ultrasonic vibration is employed in the bonding of a metallic wire onto the bonding pad, passivation layer or dielectric layer surrounding the bonding pad regions may crack due to stress concentration. In addition, the difference in coefficient of thermal expansion (CTE) between epoxy resin and the silicon chip during subsequent packaging may cause a further widening of the cracks already formed in the passivation layer or the dielectric layer.
One method of reducing uneven stress distribution within a semiconductor package is to form a plurality of bonding pad metallic layers in the desired bonding pad locations. The bonding pad metallic layers are similar in shape to bonding pads during interconnect fabrication. Plugs having a circular, rectangular or other shape arranged in a pre-defined array pattern are used to connect between the bonding pad metallic layers or the bonding pad metallic layer and the bonding pad. The bottommost layer also connects with the silicon substrate so that stress is evenly distributed over the entire wafer by the silicon substrate. Hence, the silicon wafer is less vulnerable to damages during subsequent processing.
Conventionally, a metallic plug is formed by conducting a plasma etching operation to remove a portion of the insulation layer and expose a portion of the bonding pad metallic layer and then refilling the opening with a metal. During a plasma etching operation, a portion of the electrical charges in the plasma may be transferred to the bonding pad metallic layer. These electrical charges may be transmitted to the devices via the conductive path between the bonding pad metallic layer and the devices.
In addition, a fixed number of plugs must be provided between the bonding pads and the bonding pad metallic layers or between the bonding pad metallic layers to ensure sufficient mechanical support for the bonding pads. However, the conductive current transmitted to the devices resulting from electric charges in the plasma is proportional to the number of plugs used. In other words, the larger number of metallic plugs used, the larger will be the total amount of electric charges collected by various bonding pad metallic layers. The flow of a large conduction current into the device may lead to device failure and a lowering of production yield.
Nevertheless, reducing conduction current by using fewer plugs between the bonding pad and the silicon substrate often leads to a drop in mechanical strength of the bonding pad. Damages rendered by subsequent processing may result in a higher production cost.
A method capable of increasing mechanical strength of the bonding pad without increasing corresponding conductive current is unavailable at present. Hence, only a compromised solution involving a balance between an acceptable conduction current, a minimum mechanical support for bonding pad and production cost can be sought.
Accordingly, one object of the present invention is to provide a bonding pad structure having a detached current conduction structure and mechanical support structure. The detached current conduction structure and mechanical support structure reduces the quantity of electric charges flowing to devices during etching but increases the mechanical strength of the bonding pad. Ultimately, product yield is increased and failure rate of subsequently processed silicon wafer is reduced.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a bonding pad structure. The bonding pad structure includes independently built current conduction structure and mechanical support structure between a bonding pad layer and a substrate. The current conduction structure is constructed using a plurality of serially connected conductive metallic layers, each at a different height between the bonding pad layer and the substrate. The conductive metallic layers connect with each other via a plurality of plugs. At least one of the conductive metallic layers connects electrically with a portion of the device in the substrate by a signal conduction line. The mechanical support structure is constructed using a plurality of serially connected supportive metallic layers each at a different height between the bonding pad layer and the substrate. The supportive metallic layers connect with each other via a plurality of plugs. Furthermore, the mechanical support structure connects with a non-device section of the substrate so that stresses on the bonding pads are distributed evenly through the substrate.
In this invention, since cross-sectional area of the conductive metallic layer in the current conduction structure is smaller than the bonding pad layer, the number of plugs connected to various conductive metallic layers can be reduced. Hence, current transmitted to the device via the current conduction structure will not exceed the permitted charge current during plasma etching. In other words, device breakdown due to excess charge flow is prevented leading to a higher yield and a lower production cost.
In addition, mechanical strength of the bonding pads is increased because both the mechanical support structure connected to the bonding pad layer and the current conduction structure are used in this invention. Since the mechanical support structure and the current conduction structure are connected together via the bonding pad layer only, electric charges absorbed when forming the plugs above the supporting metallic layer will not transmit to the current conduction structure. In this way, plug density can be increased to improve supportive strength of bonding pads. In the meantime, excessive current flowing to devices leading to device failures can be prevented.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.